1. Field of the Invention
This invention relates generally to a control device for controlling the rotation of a dental air bearing turbine and more particularly to a control device for controlling the rotation of an air bearing turbine designed to apply a braking force corresponding to a fixed amount of air to the air bearing turbine by use of an compressed air actuated timer and a closing valve.
2. Prior Art
A dental handpiece (hereinafter referred to as a handpiece) using a static pressure type air bearing is designed to journal a rotating shaft carrying a cutting tool by the static pressure air bearing and to rotate the cutting tool by blowing compressed air through an air nozzle onto the front of a turbine provided integral with the rotating shaft, and has heretofore employed means for stopping the supply of compressed air because the smallness of the bearing in size and capacity makes it impossible for the bearing to be subjected to a large braking load when stopping the rotation of handpiece. However, the control means of the kind described is not free from the disadvantage that the bearing is reduced in its performance as soon as the supply of compressed air is stopped but the rotating shaft and turbine continue rotating by inertia, say, for about 12 to 15 seconds and that because this inertial rotation of the turbine acts as a fan on the bearing, the interior of the bearing mechanism is subjected to negative pressure and the air outside the bearing mechanism is sucked along with the chippings powder, dust or the like suspending in the air into the bearing surface to thereby stain or roughen the bearing surface which requires very fine maintenance, resulting not only in the reduction of bearing performance but also in hurting a dental operator by possible contact with the cutting tool during inertial rotation. Accordingly, it is necessary that stop of rotation of the cutting tool should minimize the load on the bearing and should be able to be effected in the shortest possible period of time and that there should prevail no negative pressure inside the bearing mechanism. A detailed description is given of the known control device for handpiece (refer to Japanese Patent Publication No. 31519/1971) that meets such requirements with reference to the control circuit shown in FIG. 1. As apparent from FIG. 1, there are disposed three-way magnetic valves 4 and 5 within handpiece 1, the magnetic valves being connected by an air feed passage 2 and an exhaust passage 3, and the magnetic valve 4 includes passages 6, 7 and 8, a first valve not shown, a spring 10 and an electromagnetic coil 11.
The passage 6 communicates through a pipe 12 with a compressed air source 9, the passage 7 communicates with the air feed pipe 2, and the passage 8 communicates with the open air, the first valve normally blocking the passage 6 by the resilience of a spring 10. Accordingly, the passages 7 and 8 normally communicate with each other. However, when the coil 11 gets excited, the first valve is sucked and moved against the action of the spring 10, and as soon as the passage 6 is opened, the passage 8 is blocked and the passage 6 is brought into communication with the passage 7. A magnetic valve 5 is the same in structure as a magnetic valve 4 and comprises three passages 13, 14 and 15, a second valve not shown, a spring 17 and an electromagnetic coil 18. The passage 13 is connected to a pipe 19 connecting to a pipe 12 while the passages 14 and 15 communicate respectively with an exhaust passage 3 and with the open air. Since the second valve normally blocks the passage 13 by the spring 17, the passages 14 and 15 normally communicate with each other. However, when the coil 18 gets excited, the second valve is sucked and moved against the resilience of the spring 17 and as soon as the passage 13 is opened, the second valve blocks the passage 15, with the result that the passages 13 and 14 are brought into communication with each other.
When a switch 20 is operated in the state shown, a contact-maker 21 is brought into contact with a contact 23, and when the coil 11 gets excited, compressed air is supplied to an air supply passage 2 to thereby rotate a cutting tool. The air after having been used is exhausted to the open air through an exsaust passage 3, passages 14 and 15. When the contact-maker 21 is brought into communication with the contact 23, a coil 26 gets excited and capacitors 24 and 25 are charged. When the switch 20 is returned to its original state, the contact-maker 21 is also brought into contact with a contact 22, the coil 11 gets deexcited, an air feed passage 2 is brought into communication with a passage 8 to thereby communicate with the open air. And simultaneously therewith, a coil 18 is excited through an already closed main contact 28 of a delay relay 30, so that compressed air is supplied from a passage 19 to an exhaust passage 3 to thereby suddenly stop rotation of the cutting tool, which is being rotated by inertia and maintain the pressure in the bearing mechanism of a handpiece 1 higher than the open air. The electric current discharged from capacitors 24 and 25 is reduced in a certain period of time and the coil 26 of a delay relay 30 is deexcited to thereby return the relay 30 to its original state, with the result that the coil 18 is deexcited and the valve 5 is returned to its original position to thereby finish operation of the control device. However, in the rotation control device for the handpiece of the kind described above a delay relay 30 operated by the charging action of capacitors 24 and 25 is used. Accordingly the capacitors and advance in deterioration by repeated use and are widely different in discharging time, depending upon an atmosphere such as a temperature and humidity, and become unstable in delay time. As a result, the handpiece does not fully stop operation, or is reversely rotated by jetting of compressed air from an exhaust passage, to thereby damage the bearing, and even when it is desired to change relay time in accordance with the strength of air pressure to be used, change of relay time is impossible without replacement of capacitors. The disadvantages and inconveniences described above are inherent in the prior art braking system of dental handpieces.